Conventionally, an electric power steering (EPS) in an automotive steering system has been proposed and become commercially practical. An electric power steering device configured to generate an auxiliary steering torque using an electric motor according to the steering torque applied to a steering wheel and transmit this auxiliary steering torque to a steering shaft is currently under development. In such an electric power steering device, the motor is connected to the steering shaft through a speed-reduction mechanism so that the rotation of the motor will be transmitted to the steering shaft after being decelerated by the speed-reduction mechanism.
Further, attention has been focused on autonomous driving technology to steer a vehicle automatically in recent years, and various techniques have been developed. When automatic steering is realized in an electric power steering device, structures having steering angle control (to calculate and control a motor electric current command value used to make the steering angle of the steering follow a desired angle) and assist control (feedback control of motor electric current to generate the torque of an auxiliary steering force accurately when the auxiliary steering force (assist) is applied to the steering mechanism by the rotational force of the motor, and the like) independently to switch between these outputs are adopted (for example, see Patent Documents 1 to 3). The steering angle control method will be described in detail. Since there is a need to detect a steering angle (absolute steering angle) of the steering in some way, the steering angle has conventionally been detected by using an angle sensor, such as a steering-wheel angle sensor, a column steering angle sensor, or a wheel steering angle sensor (for example, see Patent Document 4).